Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...

Do you need a very high quality 3.3X or 3X telecentric and do you have the ability to build it?
Here is my project for a telecentric suitable for a H-alpha Telescope f: 10 (I designed and built it for my 150/1500 mm h-alpha Istar).
There are two projects, one of a 3.3X (OSLO) and one of 3X (ZEMAX), both made with the 3 same catalog lenses (I have used Optosigma lenses, but Eksma lenses it is similar, but cheaper ).
In red there are distances between lens.

telecentrico_3.3X_SATTA.jpg (358.68 KiB) Viewed 1396 times

telecentrico_3X_SATTA.jpg (970.31 KiB) Viewed 1396 times

If someone wants to make a telecentric from 4X to 5X (with these same three lenses) for another telescope (f: 7.5 or f: 6) I can do the calculations.
I also send some pictures of what I have done.

1.jpg (456.62 KiB) Viewed 1396 times

2.jpg (417.54 KiB) Viewed 1396 times

3.jpg (70.97 KiB) Viewed 1396 times

The projects are free for personal use.

Last edited by antonello on Thu Feb 08, 2018 2:52 pm, edited 1 time in total.

Thank's.
I add only some useful information (they are written in the images, but if one does not know Oslo or Zemax can not interpret it): the first negative lens must be placed just before the telescope's focus. In a 150/1500 telescope this distance is 86 mm for 3X, 92 mm for 3.3X and 113 mm for 5X ...
These values are lower than the backfocus of commercial telescopes, so these telecentric can be used without "to cut" the telescope.

1) Please note that this telecentric was born in the amateurs astronomers field for a telescope f: 10, to bring the focal ratio to f: 30 (3X), when the telecentric 3X Baader had not yet been released.
2) As someone else has already experienced, the focal ratio f: 30, for to make an etalon work properly, it is valid for etalon solid. The telecentric is not advisable for etalon in air, such as Lunt and PST, instead it is fine for etalon Daystar (which accept larger angles than the etalon Lunt and Coronado).
3) Of course, no problem to optimize the telecentric for 393nm. For the f: 10 (3X) I do not see any problem. For the f: 5 (6X) there will certainly be problems in the optical quality out axis and also in the excessive length between the two negative lenses (about 380 mm).

Hello Maktownley
I have had the time to verify the quality of my telecentric in the Calcium line.
With Zemax in "optimization" mode I verified that the best scheme is the same as with H-alpha. Of course, the quality deteriorates slightly, but is always good for a field of 0.15 °, which corresponds to about 1 cm on the focal plane in a 150 mm f: 10 telescope.
Looking at the attached drawings, in Oslo for convenience and comparison with the reference image (3.3x H-alpha), it is possible to see, as is obvious, that for 0.393 nm the focus lengthens by 23 mm (image below), compared to the 200 mm focal length of the last positive lens. With these 3 simple lenses it is not possible to have more.
Of course, it is advisable to place the camera sensor at 200 mm, exactly as for the H-alpha
To obtain a larger field it is necessary to change the BK7 glass, but this leads to a different project ...
A similar speech is valid also for the 3x telecentric.
Bad news instead, as expected for 5X to f:6, where the quality is good only at the center of the field and it makes no sense to waste time.

I always wonder with these telecentric system the total length of the path, up to the CCD sensor: 350 to 400mm, depending on the design.
This induces torque on the focuser, especially if using a heavy Ha etalon close to the CCD.

Isn't possible to make the design shorter?
Daystar have a quite compact system with their quark filters.

Must the CCD be really positioned so far behind?
Sorry, I'm very poor in optic knowledge.

Hello
I apologize for the delay...
Speaking of my telecentric, I had written to marktownley that quality decreases a little in the calcium line ... In reality this is not true, what happens is that it is the disc of difraction that in the wavelength of the calcium is half of that in H-alpha ... but the quality is similar, with the image points inside the difraction circles.
As for the message of allhoest, I understand her need for but for this purpose it is necessary to design a system with out-of-catalog lenses and an achromatic group. The work is long and expensive, and at the same time I have a thousand other projects in waiting list. I'm sorry.

Hi,
When it come to designing telecentrics with stock lens, sometimes you need to be creative. Also the higher the power the harder it will be. For an FS78 I used two -lens to make an bi concave lens. The front side was an -200mm then a -50mm back to back with an positive meniscus lens to get to a good F/32. Other times just using all plano lens work better. The design for a Sky90 used only Edmund plano lens. Two -125mm and a +250mm. This one was a 5X telecentric. It was about 12" long with a 200mm back focus. The main thing is if you are designing a telecentric for a 60mm objective lens don't expect it to work well on a 150mm system. You need to design to what it is going to be used on for it to work its best. This is not to say that being close will still not work just fine.
Mark W.

Hi,
When it come to designing telecentrics with stock lens, sometimes you need to be creative. Also the higher the power the harder it will be. For an FS78 I used two -lens to make an bi concave lens. The front side was an -200mm then a -50mm back to back with an positive meniscus lens to get to a good F/32. Other times just using all plano lens work better. The design for a Sky90 used only Edmund plano lens. Two -125mm and a +250mm. This one was a 5X telecentric. It was about 12" long with a 200mm back focus. The main thing is if you are designing a telecentric for a 60mm objective lens don't expect it to work well on a 150mm system. You need to design to what it is going to be used on for it to work its best. This is not to say that being close will still not work just fine.
Mark W.

Hi Mark,

Interesting. I also ahve a FS-78 and a SKY 90 and was designing a few years ago a lens combination to get a f/99 light beam so I could get the full Sun into a Quantum filter.

Two weeks ago I was at NEAF in Suffern NY and talked to Jen Winter owner of Daystar and she told me f/99 is not good for Quantum filters and I shoudl go for something like f/35.

I managed to get good image quality using 4 Edmund Optics lenses.

Sky90-TC-252-271-250-243.png (16.52 KiB) Viewed 734 times

Would you care to share the lens spec of Edmund Optics and your optical drawing ?

OK, I'll dig them out. Your F/99 really would not be that much better then an F/45. The image size would be large.
If you where going for a collimated system. Then you will fine that the over all performance of the system will work it best when it is not at "infinity " but at about F/60. Then your back lens brings it to F/11-F/15.
Any way that is what I have seen playing with designs.
Mark W.

OK, I'll dig them out. Your F/99 really would not be that much better then an F/45. The image size would be large.
If you where going for a collimated system. Then you will fine that the over all performance of the system will work it best when it is not at "infinity " but at about F/60. Then your back lens brings it to F/11-F/15.
Any way that is what I have seen playing with designs.
Mark W.

Hi Mark,

The idea of f/99 has been discarded after I talked in April at NEAF 2018 with Jen from Daystar.

BTW the prototype I made for my SKY 90 500mm focal length did produce at the output a bit larger image.

RS-TeRe-reworked.jpg (399.66 KiB) Viewed 688 times

RS-TeRe-02.jpg (1.07 MiB) Viewed 688 times

below a daylight image taken through a IR 742nm filter ( lets evrything through over 742nm) It is an Astronomik Pro Planet filter

Hi Group,
For the FS78, I have the Zmax design and the one I did with OSLO edu. The Sky 90 was done with Zmax and used Thor lens.
Images shot with the sky 90 telecentric where still quite good. Even thou it looks like your pushing the limit on the spot diagram.
Mark W.